1. Field of the Invention
The present invention relates to integrated circuit semiconductor diodes and transistors.
2. Prior Art
Semiconductor devices tend to be divided into discrete components and integrated circuits. The discrete devices include single function components such as bipolar transistors, junction field effect transistors, surface field effect transistors, silicon controlled rectifiers, etc. and some integrated components such as insulated gate bipolar transistors. One characteristic that is common to all the discrete components is the lack of external power supply requirements.
Recently a new form of discrete circuit has entered the market; a highly efficient diode made from surface field effect transistors, an integrated circuit diode (ICD). This circuit in its present form (passive form) does not utilize any on-chip drive circuitry; however, with the addition of either external or internal power, these circuits can improve their performance dramatically by utilizing on-chip circuitry to actively drive the transistor gates (active form).
Utilizing external power for this purpose tends to be less attractive because of the added circuit board complexity. However, it does have the advantage of not altering the external signal while drawing the charge needed for the onboard supply voltage. In most applications, the added convenience of the self-powered circuit would be advantageous.
In typical semiconductor diodes, conduction in the forward direction is limited to leakage current values until the forward voltage bias reaches a characteristic value for the particular type of semiconductor device. By way of example, silicon pn junction diodes don't conduct significantly until the forward bias voltage is approximately 0.6 to 0.7 volts. Many silicon Schottky diodes, because of the characteristics of the Schottky barrier, can begin to conduct at lower voltages, such as 0.4 volts. Germanium pn junction diodes have a forward conduction voltage drop of approximately 0.3 volts at room temperature. However, the same are rarely used, not only because of their incompatibility with silicon integrated circuit fabrication, but because of temperature sensitivity and other undesirable characteristics thereof.
In some applications, diodes are used not for their rectifying characteristics, but rather to be always forward biased to provide their characteristic forward conduction voltage drop. For instance, in integrated circuits, diodes or diode connected transistors are frequently used to provide a forward conduction voltage drop substantially equal to the base-emitter voltage of another transistor in the circuit.
In circuits that utilize the true rectifying characteristics of semiconductor diodes, the forward conduction voltage drop of the diode is usually a substantial disadvantage. By way of specific example, in a DC to DC step-down converter, a transformer is typically used wherein a semiconductor switch controlled by an appropriate controller periodically connects and disconnects the primary of the transformer with a DC power source. The secondary voltage is connected to a converter output, either through a diode for its rectifying characteristics, or through another semiconductor switch. The controller varies either the duty cycle or the frequency of the primary connection to the power source as required to maintain the desired output voltage. If a semiconductor switch is used to connect the secondary to the output, the operation of this second switch is also controlled by the controller; one form of this switch configuration circuit is called a synchronous rectifier.
Use of a semiconductor switch to couple the secondary to the output has the advantage of a very low forward conduction voltage drop, and has the disadvantage of requiring careful timing control throughout the operating temperature range of the converter to maintain the efficiency of the energy transfer from primary to secondary. Timing of the switching action for the primary versus the secondary is critical and must take into account the phase delays of the transformer and other elements. These circuits are obviously very costly.
The use of a semiconductor diode for this purpose has the advantage of eliminating the need for control of a secondary switch, but has the disadvantage of imposing the forward conduction voltage drop of the semiconductor diode on the secondary circuit. This has at least two very substantial disadvantages. First, the forward conduction voltage drop of the semiconductor diode device can substantially reduce the efficiency of the converter. For instance, newer integrated circuits commonly used in computer systems are designed to operate using lower power supply voltages, such as 3.3 volts, 3 volts and 2.7 volts. In the case of a 3 volt power supply, the imposition of a 0.7 volt series voltage drop means that the converter is in effect operating into a 3.7 volt load, thereby limiting the efficiency of the converter to 81%, even before other circuit losses are considered.
Second, the efficiency loss described above represents a power loss in the diode, resulting in the heating thereof. This limits the power conversion capability of an integrated circuit converter, and in many applications requires the use of a discrete diode with a heat sink of adequate size, increasing the overall circuit size and cost. Obviously any improvement in the forward voltage drop will have a major impact on the overall circuit performance.
Another commonly used circuit for AC to DC conversion is the full wave bridge rectifier usually coupled to the secondary winding of a transformer having the primary thereof driven by the AC power source. Here two diode voltage drops are imposed on the peak DC output, making the circuit particularly inefficient using conventional diodes, and increasing the heat generation of the circuit requiring dissipation through large discrete devices, heat dissipating structures, etc. depending on the DC power to be provided.
Therefore, a semiconductor diode having a low forward conduction voltage drop would be highly advantageous to use as a rectifying element in circuits wherein the diode will be subjected to both forward and reverse bias voltages from time to time. While such a diode may find many applications in discrete form, it would be further desirable for such a diode to be compatible with integrated circuit fabrication techniques so that the same could be realized in integrated circuit form as part of a much larger integrated circuit. Further, while reverse current leakage is always undesirable and normally must be made up by additional forward conduction current, thereby decreasing circuit efficiency, reverse current leakage can have other and more substantial deleterious affects on some circuits. Accordingly, it would also be desirable for such a semiconductor diode to further have a low reverse bias leakage current.
The ICD in its passive form provides lower forward voltages than Schottky diodes, with enhanced reliability at a competitive price. They also provide an attractive alternative for the higher voltage portion of the synchronous rectifier market; however, they are not able to replace the entire synchronous rectifier market.